1. Field of the Invention
The present invention relates to chip antennas and antenna devices. In particular, the present invention relates to a chip antenna and an antenna device used in mobile communication and mobile communication devices for local area networks (LAN).
2. Description of the Related Art
FIG. 12(a) is a plan view of a conventional chip antenna and FIG. 12(b) is a cross-sectional view taken along section line A--A of FIG. 12(b). This chip antenna 1 is of a microstrip type and is provided with a radiation electrode 3 as an antenna element on a main surface of a planar dielectric substrate 2 and a ground electrode 4 on the other main surface of the substrate 2. The dielectric substrate 2 is a planar rectangular member comprising a dielectric ceramic material such as aluminum or a polymeric compound. The radiation electrode 3 is smaller than the dielectric substrate 2, whereas the ground electrode 4 is formed on the entire main surface of the dielectric substrate 2. The ground electrode 4 is connected to an external conductor 6 of a coaxial cable 5 and the radiation electrode 3 is connected to a central conductor 7 at the feeding point 8.
The resonance frequency f and the bandwidth BW of the chip antenna 1 are determined by the following equations in response to the shape of the antenna: EQU f=Co/2.times.(e).sup.1/2 .times.l (1) EQU BW=(K.times.d.times.f)/.epsilon. (2)
wherein Co is the velocity of light, .epsilon. is the relative dielectric constant of the dielectric substrate 2, l is a vertical length of the radiation electrode 3 as the antenna element, K is a constant of proportionality, and d is the thickness of the dielectric substrate 2 shown in FIG. 12(b).
When the resonance frequency f is constant, use of a material having a large relative dielectric constant as the dielectric substrate 2 is capable of reducing the vertical length l of the radiation electrode 3, thus miniaturizing the chip antenna 1.
When the resonance frequency, however, is constant in the above-mentioned conventional antenna, a miniaturized antenna having a large relative dielectric constant has a narrow bandwidth and is not suitable for mobile communication devices which requires a broad bandwidth. Miniaturization of the antenna is therefore barely compatible with a broad bandwidth.